Biaxially oriented polyester films have excellent mechanical, thermal and electrical properties and chemical resistance, and are used in various fields. Particularly, they are suitably used as base films for magnetic tapes. As the characteristics of the base film for magnetic tapes strongly influence the quality of the manufactured magnetic tape, demand for base films of higher quality has been increasing steadily in recent years along with progress in magnetic tape technology. Base films provided with further improved flatness are wanted in order to improve such electromagnetic characteristics as video S/N, chroma S/N, dropout, generation of envelope, etc. of video magnetic tapes, for instance.
On the other hand, base films which are simultaneously provided with improved slipperiness, durability and surface flatness are desired for improving the appearance of wound tapes and sliding property, as well as wear resistance.
In conventional base films, however, there has been a problem that if the number of minute protrusions formed on the surface of films and the height thereof are increased in order to improve the sliding property and durability of films, the surface flatness is impaired resulting in deterioration in electromagnetic characteristics. In order to overcome this problem, an attempt has been made to provide the film surface with roughness formed by protrusions accompanied by recesses. (Refer to Japanese Laying-Open Patent Publication Nos. 66936/82, 167215/82, 167216/82, 189822/82, and 116173/74.) However, it has been pointed out that these films are inferior in durability in uses where the tapes are subject to repeated sliding under severe hot and humid conditions (Japanese Laying-Open Patent Publication No. 167,216/82).
With the intention of solving the above-mentioned problems, we made an intensive study and found that films provided with excellent sliding property and durability as well as surface flatness can be manufactured if certain conditions are satisfied in the relation between the center line average roughness R.sub.a (.mu.m) and the number of secondary interference fringes measured by the multiple interference method H.sub.2 (number/mm.sup.2), and we made this invention.